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Hello, this is my first post on this site. I have been a BMW tech for nearly thirty years at both dealer and independent shops. Have an E53 X5 with 4.4 N62 at the shop giving us fits. The booster, master cylinder and vacuum hose with check valve have been replaced. It has been bled three times, including using Autologic software to activate the pump in the ABS system. The pedal is HARD AS A ROCK with the engine shut off, but is mushy when engine running. It will sink to within an inch of the floor with engine running. Driving the car, the soft pedal will almost throw you into the windshield when applied at normal pedal pressure. In simulated panic stops, ABS works and car stops dead straight. Like I said before, pedal is high and hard with engine off, but feels like it is getting WAY TOO MUCH vacuum assist. Has anyone out there heard of this? Customer patient and has spent considerable funds on this problem but it's not right yet. HELP!

I have had exactly the same problem for a number of years with my 2004 BMW 545i and it has never been resolved. In addition to the items and work you performed, I also had the DSC unit replaced by my BMW dealership. The brakes were perfect when the car was first purchased.

I am at a complete loss and understand your frustration. As I am not sure if the BMW scan tool was properly used to calibrate the brake pressure, steering angle etc. I will ask for this to be repeated. If this still does not solve the problem, I may consider replacing the vacuum pump though I am not convinced this is the root cause of the problem.

What sort of vaccuum reading do you get at the booster, between the check valve and the booster nipple and is there any evidence of oil?
The N series engines use an engine driven vacuum pump that pukes oil into the boosters; there was a recall for this. But that's usually a loss of boost, not an overboost.

Anything weird at the pedal, or the pivot? There was some flavor of recall for the pedal a while ago, but I doubt that applies to the N62 X5s.

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Quote:

Originally Posted by Fast Bob

JUST...LOOK...UNDER...THE...CAR....for Chrissakes....it`s like checking to see if a dog is male or female....

Quote:

Originally Posted by smolck

A bimmer with forced induction should have a proper manual gearbox. Anything less is like french kissing your sister.

I did not check the vacuum but assume this is good as I am getting brake assist and there is no evidence of oil. The check valve recall was performed before the soft pedal problem several years ago. Prior to the check valve recall, the pedal was hard after sitting overnight and was normal with the engine running. Perhaps this is related.
Based on my research, apparently there is a check valve in the vacuum pump but this fails and BMW sustitutes a check valve in the vacuum line going to the booster. I cannot verify this however.